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Topic: How to Tune for Fuel Economy (Read 9171 times)

I have a 85 Sedan with the 258. I have done the ECM Test Bypass, Motorcraft Distributor cap, new plugs and wires, Petronix coil, rebuilt BBD stepper motor carb, torqued intake bolts, new EGR valve, choke operates correctly, timing set to 9 degrees at idle. It has 2.35 gears. It pulls a solid smooth 20 psi vacuum at idle. I just did a 400 mile trip and only got 18mpg. What else should I do to get better gas mileage?

You could install a vacuum gauge and try to keep the needle as high as possible. The more throttle, the less vacuum, the lower the economy, and the gauge will show it. Some vacuum gauges in the 70's were designed with economy markings on them. A vacuum gauge would also prove useful if the engine developes a running problem. Good diagnostic tool. If you do mount a vac gauge in the car, consider sticking it somewhere that you can see it while not taking your eyes off the road, like the windshield pillar.

Also, tire size, tire type, and tire pressure play a huge role in fuel economy. You might shop for a lighter wheel/tire combo.

18mpg isn't bad for a non-overdrive 4x4 with a big honkin inline six engine. Try to get that out of a 4x4 XJ with a 4.o and an automatic. At least the Jeep has OD, but it'll still barely make 15mpg at best.

Swap in a OD 5-speed manual to see improvements in economy . . and to make it more fun to drive.

It's an automatic with factory size tires and 2.35 gears. We live in a very hilly area. I ran at 55 mph. 25% hilly highway driving the rest was flat highway. Some are saying they are getting 24 on trips. Just want to make sure the engine is setup right to get the best fuel economy. I do run vacuum gauge. On the flat highway I was pulling around 12psi steady and smooth needle. Does a Motorcraft carb get better economy? What is the best way to determine if I have a clogged CAT?

The ones I have seen with high MPG numbers had properly working Carters. They are supposedly a really good carb if you can figure them out. I've had 2 Eagles with the 6. My first one, when I lived in CO, had a 4 speed with 2.35's. The best I ever got was 16 mpg. My current one ,when I first got it, had an auto and 2.73's. I would get 13-15 mpg. I had the carb rebuilt with no success. I installed a 4.0L head, Comp cam camshaft and Motorcraft carb and 3.07 gears. It had A LOT more power and would get 21mpg. Then I added A/T tires, roof rack and heavy custom bumpers and I'm down to 16 mpg. I have an AW4 waiting to go in and plan on installing 3.55 gearing.

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"I know he'd be a poorer man if he never saw an eagle fly,Rocky mountain high" John Denver

My Concord has 2.53 gears with an automatic and carter carb with feedback system hooked up and functioning, as well as all original emission components hooked up. I have also done the MIU upgrade to pertronix coil and have a pcv modification. Before I rebuilt the carburetor, I would get about 12 mpg city, and the best I ever got highway was just over 19 mpg. I rebuilt the carb over the summer, and I believe I get about 14-16 mpg city, and now I can get over 26 mpg highway. However, when I got over 26, I was going 65 mph and was behind trucks for a good amount of the drive. A few weeks ago, I did the same drive, and got over 23 mpg, and this was at 76 mph. Speeding over 65 really takes a toll on the gas mileage. When I rebuilt the carb, I set the fuel mixture towards the leaner side hoping for better mileage. The carb gaskets were also slightly leaking before I rebuilt it, that's probably where the increase in mileage came from.

Does anyone have any experience with how to setup the tune for max MPG on a 1985 258.

I have rebuilt the Carter BBD feedback carburetor. I believe I have the choke, choke pulloff, accelerator pump and enrichment needles all set right. If I set the stepper motor towards the lean side it stumbles badly. It doesn't seem to have any vacuum leaks (pulls steady 18psi at idle, 12psi flat highway). The choke kicks out about 2 minutes down the road so that seems right. It seems to me like this motor has to work so hard to make it up hills that it will never be able to get good MPGs. I wonder if good MPG only happen in flat areas. 30 mpg highway is really one of the main appealing things about this AWD car. Otherwise I would have found a Cherokee. A Cherokee has more room and is a better AWD vehicle. If I can't get at least 25 MPG highway I will be selling this car. I used to drive a 69 Riviera (about 5000 lbs) with a 455 that got 14mpg on the highway. How can a car this small with an engine half the size only get 18 on the highway.

I have owned a couple of Cherokees, they will NOT get 30 MPG, try about half of that

An Eagle is a very heavy car, good if someone runs into you, bad if you are trying to obtain maximum gas mileage

If you can get 20MPG on the highway with an Eagle 6 cly auto, you are about where you re going to be short of adding things like overdrive transmission or taking a lot of stuff off the car to make it lighter

I have the window sticker for mine, it says 16 is the av fuel economy, and thats when it was NEW!

The only AMC i ever had that got 30 MPG was my 81 Spirit with a 4 cyl stick, no air, no PS, no PB, no nothin to steal HP and economy, and that was on relatively flat road interstate driving at 60MPH (it wouldnt go much faster than that!)

My 78 Concord AMX 258 Auto gets 17-18 around town, havent been on a trip yet, but its a lot lighter than an eagle and not AWD of course

If you have a working stepper motor carb, I think having the ECM working is the best for economy. That motor tries to keep the carb in tune without you. It adjusts for everything but altitude since carbs are volumetric devices. It's very similar in concept to a TBI setup.

If you're set on not using the ECM, you can get or build a GM TBI to replace the carb. You can get a setup new for around $1,000, or 2-3x a good non-stepper BBD or about twice a Webber 32/36 kit.

In cost/benefit I like the TBI unless you're going to put a whole 4.0 in and have a donor vehicle available.

The Eagle is also a bit... brickish. Putting a light rock plate under the engine, adding a front air dam spoiler, and putting wheel spoilers on will improve highway fuel economy.

Off road tires can eat a couple of mpg, find some 195/75, or at most 205/75 tires and plain steel wheels to fit if you have some off road clodhoppers on. Narrower tires are also better in the snow. Moving down to some very common 195/65R15s may make a difference in the city, you may have to adjust the speedo gear a tooth to match and I'd only consider it with a 2.35 geared car.

If you have a working stepper motor carb, I think having the ECM working is the best for economy. That motor tries to keep the carb in tune without you. It adjusts for everything but altitude since carbs are volumetric devices. It's very similar in concept to a TBI setup.

I was thinking the same thing. Taking the ECM out of the system will make the stepper and distributor adjustment inoperable. My '83 got an average of 21 MPG on it's first road trip in stock form. I since did the ECM test bypass (made a wiring adapter, so no cutting/splicing of wires) and the "big cap" ignition upgrade (from GRONK), and now the car gets less MPG. It does have a LOT more power than before, but the MPG is suffering. I think I am going to remove the wiring adapter, plug the stepper back in, and see if the MPG increases. I believe it will. Just having the higher voltage coil, cap and wires, will help even with the ECM and stepper hooked up.

I would agree that taking the timing control and stepper operation out would most likely result in less MPG. However, to have those functioning, your almost 30 year old components, hoses and wires under the hood have to be in proper working order for those to operate properly. To get the carb set up correctly, a dyno with a competent operator could set your carb where your O2 levels need to be for optimum MPG (if you can find someone who will touch the Carter). However, you mentioned hills. With a 235 gear ratio, I don't think you will do very well if you need to ascend hills often in your commute. Everything has to come together, carb, engine health, igniton, tires, aerodynamics, proper gearing, etc. to achieve the highest MPG. I knew I would never get decent MPG's out of my Eagles, so I bought a cheap Ford Festiva that gets 40 mpg for my daily commute. I have just put 30,000 miles on it since I got it almost two years ago and figure I have saved about $2200 in fuel after deducting what I paid for it, maintenance and insurance. Not to mention the savings in maintenance on the Eagles, such as tires and other wear.

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"I know he'd be a poorer man if he never saw an eagle fly,Rocky mountain high" John Denver

I hear ya on the Festiva. I'm no into owning allot of cars. That's why I want to get this one working. I really like the challenge. I like tweaking. It just seems like I have done allot of the things suggested with no improvement. I do want to try the ECM system and see what happens. Not sure what I have to have hooked up for it to work. I know I have to undo the bypass and hookup the stepper motor. What else do I need to have hooked up?

The first "modification" I do to every AMC I have ever owned is change the ignition coil to match a late '70s Mopar. (NAPA number IC12 or IC12SB the cheaper version) and do away with the horseshoe connector that left me stranded several times. It makes nearly double the spark of the stock coil, which was designed for points systems. You need hotter spark to jump a lean gap on the plugs.

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FOR SALE'87 Comanche 4.0L AW4/NP242 3.73 gears, lifted 5" on near new 33" tires $3500 obo'82 Eagle Wagon (Brown Betty) 4.2L/727/NP229 and soon 8.25 rear axle and non-vacuum 3.08 front to match. 235/75/15 tires. It's down to minor body work (someone creamed my driver's door but I have another in good shape) almost rust free and interior work (seats are worn and carpet torn). $2000 but about to go up due to more work being done.

for fuel economy i would recommend bosch fusion spark plugs 4508 or platinum +4 4428, as well as replacing the tired fuel pump, and coil (stock coil), what condition is the pcv valve in? and fuel and air filters? and re connecting the ecm would be recommended, what exactly do you have left in your engine compartment for the electronics? does the stepper motor operate when power is applied when the carb was rebuilt?( also are the idle/milture screws set to factory spec), what is the ignition timing set to? also if you reconnect the ecm it must be reset using the correct amc proceedure due to the knock sensor, if you reconnect the ecm i would recommend a new oxygen sensor as well, also using royal purple engine oil i found helped mileage some

As someone who just ordered the Motorcraft 2150 and an HEI distributor... I don't like hearing everyone talk about having to reconnect the ECM, I was planning on permanently bypassing it.

The ECM isn't on my list of things to have. Mind you my end game plan is EFI from a Cherokee

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FOR SALE'87 Comanche 4.0L AW4/NP242 3.73 gears, lifted 5" on near new 33" tires $3500 obo'82 Eagle Wagon (Brown Betty) 4.2L/727/NP229 and soon 8.25 rear axle and non-vacuum 3.08 front to match. 235/75/15 tires. It's down to minor body work (someone creamed my driver's door but I have another in good shape) almost rust free and interior work (seats are worn and carpet torn). $2000 but about to go up due to more work being done.

The Chrysler HO EFI conversion kit? No longer available, but a '91 to '95 XJ setup is pretty easy to convert into the Eagle, especially if you are doing the head swap anyways. I have a J10 with 4.0L/AW4 and we converted my Spirit of the Eagle to 4.2L/AW4/NP242. He plans to switch to EFI later, but the 4.2L gets 20 mpg already with the non-feedback BBD I modified on it.

The best part about that kit was the MPFI intake designed for the 4.2L head that clears the brake booster. I plan on an Astrovan Hydroboost to cure that issue.

« Last Edit: January 14, 2013, 07:51:43 PM by carnuck »

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FOR SALE'87 Comanche 4.0L AW4/NP242 3.73 gears, lifted 5" on near new 33" tires $3500 obo'82 Eagle Wagon (Brown Betty) 4.2L/727/NP229 and soon 8.25 rear axle and non-vacuum 3.08 front to match. 235/75/15 tires. It's down to minor body work (someone creamed my driver's door but I have another in good shape) almost rust free and interior work (seats are worn and carpet torn). $2000 but about to go up due to more work being done.

I just had a strange thing happen. I filled up and topped off after a 110 mile mostly 55 mph run, about 20% hilly. I got 25 mpg, Sweet. I'll keep you posted. I think it may be as a result of an adjustment to the metering rod piston position. I loosened the small flat head screw on the pump counter shaft, light pushed down on the metering rod assembly while lightly pressing down on the tab near the flat head screw and then I tightened the flat head screw. It seems to be running better now. Time will tell.

It amazes me that so much un-burned gas is being wasted on so many of these engines. How can an engine run so rich and still run decent? Strange.

Also on the PCV valve. I have been experimenting. I tried hooking up 2 PCV valves (some say it works better on and older 258). So far this has lead to increased oil usage, difficult tuning and an erratic idle. I think it is bleeding to much air with two. I switched back to the correct setup with 1 PCV valve from the front opening on the valve cover with the evaporator tank teed in and an open connection from the rear opening on the valve cover to the air cleaner. It seems to run best this way. This engine doesn't seem to have a big blowby problem.

Thanks for all of the posts!!!!!! Hopefully it will help allot of our fellow nesters.

It's available again? Cool, but $till too expen$ive though. EPA put it on hold in '05 I was told.

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FOR SALE'87 Comanche 4.0L AW4/NP242 3.73 gears, lifted 5" on near new 33" tires $3500 obo'82 Eagle Wagon (Brown Betty) 4.2L/727/NP229 and soon 8.25 rear axle and non-vacuum 3.08 front to match. 235/75/15 tires. It's down to minor body work (someone creamed my driver's door but I have another in good shape) almost rust free and interior work (seats are worn and carpet torn). $2000 but about to go up due to more work being done.

Here's an interesting idea. According to info I have been reading if the coolant temp switch is unplugged the ECM does not operate. Would that be EASIER than the bypass. Also a dashboard switch could be connected to the coolant temp switch connector and then you could turn the computer off or on when ever you want to. Hmmmm, might be fun.

Here's an interesting idea. According to info I have been reading if the coolant temp switch is unplugged the ECM does not operate. Would that be EASIER than the bypass. Also a dashboard switch could be connected to the coolant temp switch connector and then you could turn the computer off or on when ever you want to. Hmmmm, might be fun.

Don't think it would work right. Computer may not go into closed loop until the coolant is warmed up, but I believe that it always controls the ignition timing. And if the coolant is reading cold all the time, timing will be retarded giving poor economy.

Still, I don't know why I hadn't considered putting in a relay for the wires for the distributor and the stepper motor... seems do-able.

You could do that for the electronic portion of going between bypassed and unbypassed. But you would still have to switch the distributor vacuum hose, and reset the ignition timing every time you switched back and forth.

Here's the link. Maybe this guy doesn't know what his talking about. If he does; then one of the main things that switched the computer on and off is the coolant temp switch and the air temp switch. So unplug the air temp switch (open) and run a switch to the unplugged coolant temp sensor connector. That would switch it off and on. Hmmmm

It would sort of make the ECM Test a bit silly, cramming extra electronics in afterward... but on the other hand it would be interesting to see...Difficult, and more work than the alternative in all honesty, but interesting to ponder none the less.

You could have a relay control the path to the distributor/ecm test bypass, and it could easily control an electronic vacuum switch for the carb, but I can't think of how you could automate the position of the stepper motor or the use of an electronic ignition controller to retard the timing...

Maybe such a thing exists, a standalone "two position/ two stage electronic ignition controller/ICU" that would allow you to switch between calibrated timings on the fly... if it doesn't it stands to reason that someone could make some money on such a thing... (though likely not in this crowd, at least, I don't think I'd want one.)

The real question is how easy would it be to tune the EI system to work with the existing ECM and COULD we find a way to easily have two stored timings that could easily be activated by the relay, and would it be possible to automate the position of the stepper motor? (seeing as it's a stepper motor it seems reasonable... but how would you sense the position to properly correct it?)

It would sort of make the ECM Test a bit silly, cramming extra electronics in afterward... but on the other hand it would be interesting to see...Difficult, and more work than the alternative in all honesty, but interesting to ponder none the less.

You could have a relay control the path to the distributor/ecm test bypass, and it could easily control an electronic vacuum switch for the carb, but I can't think of how you could automate the position of the stepper motor or the use of an electronic ignition controller to retard the timing...

Maybe such a thing exists, a standalone "two position/ two stage electronic ignition controller/ICU" that would allow you to switch between calibrated timings on the fly... if it doesn't it stands to reason that someone could make some money on such a thing... (though likely not in this crowd, at least, I don't think I'd want one.)

The real question is how easy would it be to tune the EI system to work with the existing ECM and COULD we find a way to easily have two stored timings that could easily be activated by the relay, and would it be possible to automate the position of the stepper motor? (seeing as it's a stepper motor it seems reasonable... but how would you sense the position to properly correct it?)

(Besides my "Gronk kit" is scheduled to arrive in the mail tomorrow, so the sooner I sleep, the sooner it's here)

I really don't want to sidetrack this thread further, but honestly, you are WAY over thinking this. The Eagle already has an electronic distributor and a mighty fine one at that. You can send your distributor out to a competent distributor shop and have it recurved for your needs and run an MSD box for maximum spark. If you really wanted to ditch the computer but keep a carb and have modern ECU type control of your ignition, I would look into a Megajolt Lite Jr. I was going to go this route a few years ago but then decided to go with EFI, which I still haven't done. It does away with the distributor for spark but you will have to keep it to run your oil pump. http://www.autosportlabs.net/MegaJolt_Lite_Jr.

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"I know he'd be a poorer man if he never saw an eagle fly,Rocky mountain high" John Denver

Either way what Donny is wishing he could do is pretty much impossible; although the ECM Test is easily reversed, without lots of extra junk and plenty of initial setup you would still have to pop the hood, there's no ECM Test on the fly...

I still can't think of a way to sense the position of the stepper motor though, so I'm ruling it impossible at present.

I'm probably showing some age here... The Apple II 5-1/4 floppy drive used a stepper motor. When the computer booted the drive always made a rattling noise; that was the stepper motor banging the drive head against the end of its travel.

Once the stepper had reached the end of its travel then the controller just needed to keep track of movements, it didn't need to sense anything.

So, I'm guessing the ECM does exactly the same thing. They weren't designed too many years apart.

I really don't want to sidetrack this thread further, but honestly, you are WAY over thinking this. The Eagle already has an electronic distributor and a mighty fine one at that. You can send your distributor out to a competent distributor shop and have it recurved for your needs and run an MSD box for maximum spark. If you really wanted to ditch the computer but keep a carb and have modern ECU type control of your ignition, I would look into a Megajolt Lite Jr. I was going to go this route a few years ago but then decided to go with EFI, which I still haven't done. It does away with the distributor for spark but you will have to keep it to run your oil pump. http://www.autosportlabs.net/MegaJolt_Lite_Jr.

I bet it wouldn't take much to program an Arduino to replace the ECM. I don't know that anyone has replicated a computer controlled carb system before. It could be fun.

I think the last of the 4.0 engines used distributorless ignition as well. I think they used a cam sensor on a distributor base, but I've never seen one in person to know.

That same arduino board could replace the relay as well, and control the electronic vacuum switch.

Why didn't I think of that?

Have an egg!

In all seriousness if someone wanted to go through the effort of designing and making one, I'd help invest.

The only experience I have with stepper motors is a frustrating defeat... I was trying to find a way to make scrapped hard drive motors into micro-rc engines... but was never able to figure out how to effectively pulse them right, they always hit a certain speed and would start to stutter.

I initially thought it was promising, due to the high rpms, but I eventually gave up.

I really don't want to sidetrack this thread further, but honestly, you are WAY over thinking this. The Eagle already has an electronic distributor and a mighty fine one at that. You can send your distributor out to a competent distributor shop and have it recurved for your needs and run an MSD box for maximum spark. If you really wanted to ditch the computer but keep a carb and have modern ECU type control of your ignition, I would look into a Megajolt Lite Jr. I was going to go this route a few years ago but then decided to go with EFI, which I still haven't done. It does away with the distributor for spark but you will have to keep it to run your oil pump. http://www.autosportlabs.net/MegaJolt_Lite_Jr.

I bet it wouldn't take much to program an Arduino to replace the ECM. I don't know that anyone has replicated a computer controlled carb system before. It could be fun.

I think the last of the 4.0 engines used distributorless ignition as well. I think they used a cam sensor on a distributor base, but I've never seen one in person to know.

CPS on the bell and cam sensor in the hole where the dist lived. The coil packs suck because 1 would burn out and the head had cracking issues too.

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FOR SALE'87 Comanche 4.0L AW4/NP242 3.73 gears, lifted 5" on near new 33" tires $3500 obo'82 Eagle Wagon (Brown Betty) 4.2L/727/NP229 and soon 8.25 rear axle and non-vacuum 3.08 front to match. 235/75/15 tires. It's down to minor body work (someone creamed my driver's door but I have another in good shape) almost rust free and interior work (seats are worn and carpet torn). $2000 but about to go up due to more work being done.

Back to fuel economy, you may also be filling your gas tank at different rates depending on how fast the pump is flowing on a certain day. At fast pump speed, the Eagle will only take about 15 gallons because the fuel splashes up and triggers the sensor to shut off. This can be very deceiving for gas mileage unfortunately. With slower pump speeds, it will usually shut off around 19 gallons or so. Wagons have a 22 gallon tank, and the 2-door models have a 21 gallon tank. In order to fully fill your tank and get the closest possible estimate of mileage, you need to make an educated guess from the position of your gauge (provided that is also operational) and prior fill-ups. When my gauge shows 3/4 full, I know it should take anywhere from 5-6 gal. Then you have to pump as slow as humanly possible for the last 5 gallons so it won't shut off and hope that your estimate was in the ballpark.

I have a feeling this is where a lot of those "25+mpg" ratings come from (of course some are O/D Manuals). I'd say 20-21 is the best you'll get in 4wd w/auto stock, but heck that's still decent for a carb. Other things like ignition system, fuel delivery mods, weight, aerodynamics, tires, and gearing will have positive or negative effects on mileage. The Eagle has great potential for a 30+mpg 4wd vehicle, but not many have made their mods with this goal in mind.

Also fuel in areas without alcohol shoved into it get you better fuel economy roughly equal to the amount of alcohol (when they did 20% here I had to shop around for non-laced fuel)

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FOR SALE'87 Comanche 4.0L AW4/NP242 3.73 gears, lifted 5" on near new 33" tires $3500 obo'82 Eagle Wagon (Brown Betty) 4.2L/727/NP229 and soon 8.25 rear axle and non-vacuum 3.08 front to match. 235/75/15 tires. It's down to minor body work (someone creamed my driver's door but I have another in good shape) almost rust free and interior work (seats are worn and carpet torn). $2000 but about to go up due to more work being done.

So far I have found that when I get high mpg it is because of the topping off problem mentioned earlier. In a recent 700 mile trip I logged my mpg at each fill-up. I got 18.4, 16.7, 25.1, 15.6. This was all highway about 80% very flat roads. I have never had a car with that much swing in mpg so it seems like the top off problem is where the high mpg posts come from. I also made an adjustment to the stepper motor position half way through the trip. I started out very lean (backed out) then I changed it to full rich (all the way in) with really no change in mpg. So either this car will never get good mpg or I am trying to fix the wrong thing. Another thing I found on this trip was when I switched to 89 octain the engine seemed to run smoother with no hint of ping so thats what I will run from now on. Sadly that cuts into the cost per mile. I just very disappointed in this car as a daily economical driver.

Here's some interesting math. If I can improve my daily average mpg by 2 mpg I would save about $350 per year on gas (driving 250 miles per week). Since that is true if my low mpg problem is related to my Carter BBD I could replace it with a Weber or Motorcraft and pay for the new carburetor with the money I save on gas.

Lots of people tell me I'm crazy and it's unnecessary (some even claim it's "harmful", but all of the gasoline up here is between 87-90, and if they sell 93 in places in the lower 48 states, then it seems reasonable that 90 isn't going to harm it)

the carter carb will get better gas mileage than a conventional one when the system is hooked up, might want to still try hooking the ecm back up with a new oxygen sensor and resetting the timing before putting out the cash for a new carb, or even just reset the timing, and as i mentioned before the bosch plugs will improve the mileage